Periodic and aperiodic channel state information (CSI) reporting for MIMO

ABSTRACT

Certain aspects of the present disclosure provide techniques for performing periodic and aperiodic CSI reporting for MIMO operations. According to certain aspects, operations for performing periodic and aperiodic CSI reporting for MIMO generally includes configuring a user equipment (UE) that is capable of MIMO with different parameters for periodic and aperiodic channel state information (CSI) reporting, wherein the different parameters indicate at least one of what resources to measure or what information to report, and receiving periodic and aperiodic CSI reporting from the UE according to the configuration.

BACKGROUND

I. Field

Certain aspects of the disclosure generally relate to wirelesscommunications and, more particularly, to techniques for configuringmultiple-input multiple-output (MIMO) channel state information (CSI)feedback for periodic and aperiodic reporting.

II. Background

Wireless communication systems are widely deployed to provide varioustypes of communication content such as voice, data, and so on. Thesesystems may be multiple-access systems capable of supportingcommunication with multiple users by sharing the available systemresources (e.g., bandwidth and transmit power). Examples of suchmultiple-access systems include code division multiple access (CDMA)systems, time division multiple access (TDMA) systems, frequencydivision multiple access (FDMA) systems, 3rd Generation PartnershipProject (3GPP) Long Term Evolution (LTE)/LTE-Advanced systems andorthogonal frequency division multiple access (OFDMA) systems.

Generally, a wireless multiple-access communication system cansimultaneously support communication for multiple wireless terminals.Each terminal communicates with one or more base stations viatransmissions on the forward and reverse links. The forward link (ordownlink) refers to the communication link from the base stations to theterminals, and the reverse link (or uplink) refers to the communicationlink from the terminals to the base stations. This communication linkmay be established via a single-input single-output, multiple-inputsingle-output or a multiple-input multiple-output (MIMO) system.

SUMMARY

Certain aspects of the disclosure provide techniques for reportingmultiple-input multiple-output (MIMO) channel state information (CSI)feedback based on different parameters for periodic and aperiodic CSIreporting.

Certain aspects of the present disclosure provide a method for wirelesscommunications by a base station (BS). The method generally includesconfiguring a user equipment (UE) that is capable of MIMO with differentparameters for periodic and aperiodic channel state information (CSI)reporting, wherein the different parameters indicate at least one ofwhat resources to measure or what information to report; and receivingperiodic and aperiodic CSI reporting from the UE according to theconfiguration.

Certain aspects of the present disclosure provide a method for wirelesscommunications by a MIMO-capable user equipment (UE). The methodgenerally includes receiving a configuration, from a base station, ofdifferent parameters for periodic and aperiodic channel stateinformation (CSI) reporting, wherein the different parameters indicateat least one of what resources to measure or what information to report;and measuring and reporting periodic and aperiodic CSI according to theconfiguration.

Aspects of the present disclosure also include various apparatus andprogram products for performing operations in accordance with themethods described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram conceptually illustrating an example of awireless communication network, in accordance with certain aspects ofthe present disclosure.

FIG. 2 is a block diagram conceptually illustrating an example of a basestation in communication with a user equipment (UE) in a wirelesscommunications network, in accordance with certain aspects of thepresent disclosure.

FIG. 3 is a block diagram conceptually illustrating an example of aframe structure in a wireless communications network, in accordance withcertain aspects of the present disclosure.

FIG. 4 illustrates an example of an antenna array that may be used forhigh dimension multiple-input multiple-out (MIMO) communications, inaccordance with certain aspects of the present disclosure.

FIG. 5 illustrates an example of an antenna port structure in accordancewith certain aspects of the present disclosure.

FIG. 6A illustrates an example of antenna port steering, in accordancewith certain aspects of the present disclosure.

FIG. 6B illustrates an example of antenna port steering for multipleelevation domain subsectors in accordance with certain aspects of thepresent disclosure.

FIG. 7 illustrates an example of separate information reportingconfigurations for periodic and aperiodic channel state information(CSI) reporting, in accordance with certain aspects of the presentdisclosure.

FIG. 8 illustrates an example of separate resource configurations forreporting channel state information (CSI) periodically andaperiodically, in accordance with certain aspects of the presentdisclosure.

FIG. 9 illustrates another example of separate resource configurationsfor reporting CSI periodically and aperiodically, in accordance withcertain aspects of the present disclosure.

FIG. 10 illustrates examples of antenna selection options for reportingCSI periodically and aperiodically, in accordance with certain aspectsof the present disclosure.

FIG. 11 illustrates an example of resource configuration for periodicCSI reporting, in accordance with certain aspects of the presentdisclosure.

FIGS. 12-14 illustrate an example structure definitions for configuringresources for CSI reporting, in accordance with certain aspects of thepresent disclosure.

FIG. 15 illustrates an example resource configuration for legacy and3D-MIMO-capable UEs, in accordance with certain aspects of the presentdisclosure.

FIG. 16 illustrates example operations that may be performed by a basestation, in accordance with certain aspects of the present disclosure.

FIG. 17 illustrates example operations that may be performed by a userequipment, in accordance with certain aspects of the present disclosure.

DETAILED DESCRIPTION

Certain aspects of the disclosure provide techniques for configuringdifferent parameters for periodic and aperiodic multiple-inputmultiple-output (MIMO) channel state information (CSI) feedback, whichmay reduce feedback overhead for CSI reporting.

The techniques described herein may be used for various wirelesscommunication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA andother networks. The terms “network” and “system” are often usedinterchangeably. A CDMA network may implement a radio technology such asuniversal terrestrial radio access (UTRA), cdma2000, etc. UTRA includeswideband CDMA (WCDMA), time division synchronous CDMA (TD-SCDMA), andother variants of CDMA. cdma2000 covers IS-2000, IS-95 and IS-856standards. A TDMA network may implement a radio technology such asglobal system for mobile communications (GSM). An OFDMA network mayimplement a radio technology such as evolved UTRA (E-UTRA), ultra mobilebroadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20,Flash-OFDM®, etc. UTRA and E-UTRA are part of universal mobiletelecommunication system (UMTS). 3GPP Long Term Evolution (LTE) andLTE-Advanced (LTE-A), in both frequency division duplex (FDD) and timedivision duplex (TDD), are new releases of UMTS that use E-UTRA, whichemploys OFDMA on the downlink and SC-FDMA on the uplink. UTRA, E-UTRA,UMTS, LTE, LTE-A and GSM are described in documents from an organizationnamed “3rd Generation Partnership Project” (3GPP). cdma2000 and UMB aredescribed in documents from an organization named “3rd GenerationPartnership Project 2” (3GPP2). The techniques described herein may beused for the wireless networks and radio technologies mentioned above aswell as other wireless networks and radio technologies. For clarity,certain aspects of the techniques are described below forLTE/LTE-Advanced, and LTE/LTE-Advanced terminology is used in much ofthe description below.

An Example Wireless Communications Network

FIG. 1 shows a wireless communication network 100, which may be an LTEnetwork or some other wireless network. Wireless network 100 may includea number of evolved Node Bs (eNBs) 110 and other network entities. AneNB is an entity that communicates with user equipments (UEs) and mayalso be referred to as a base station, a Node B, an access point, etc.Each eNB may provide communication coverage for a particular geographicarea. In 3GPP, the term “cell” can refer to a coverage area of an eNBand/or an eNB subsystem serving this coverage area, depending on thecontext in which the term is used.

An eNB may provide communication coverage for a macro cell, a pico cell,a femto cell, and/or other types of cell. A macro cell may cover arelatively large geographic area (e.g., several kilometers in radius)and may allow unrestricted access by UEs with service subscription. Apico cell may cover a relatively small geographic area and may allowunrestricted access by UEs with service subscription. A femto cell maycover a relatively small geographic area (e.g., a home) and may allowrestricted access by UEs having association with the femto cell (e.g.,UEs in a closed subscriber group (CSG)). An eNB for a macro cell may bereferred to as a macro eNB. An eNB for a pico cell may be referred to asa pico eNB. An eNB for a femto cell may be referred to as a femto eNB ora home eNB (HeNB). In the example shown in FIG. 1, an eNB 110 a may be amacro eNB for a macro cell 102 a, an eNB 110 b may be a pico eNB for apico cell 102 b, and an eNB 110 c may be a femto eNB for a femto cell102 c. An eNB may support one or multiple (e.g., three) cells. The terms“eNB”, “base station” and “cell” may be used interchangeably herein.

Wireless network 100 may also include relay stations. A relay station isan entity that can receive a transmission of data from an upstreamstation (e.g., an eNB or a UE) and send a transmission of the data to adownstream station (e.g., a UE or an eNB). A relay station may also be aUE that can relay transmissions for other UEs. In the example shown inFIG. 1, a relay station 110 d may communicate with macro eNB 110 a and aUE 120 d in order to facilitate communication between eNB 110 a and UE120 d. A relay station may also be referred to as a relay eNB, a relaybase station, a relay, etc.

Wireless network 100 may be a heterogeneous network that includes eNBsof different types, e.g., macro eNBs, pico eNBs, femto eNBs, relay eNBs,etc. These different types of eNBs may have different transmit powerlevels, different coverage areas, and different impact on interferencein wireless network 100. For example, macro eNBs may have a hightransmit power level (e.g., 5 to 40 Watts) whereas pico eNBs, femtoeNBs, and relay eNBs may have lower transmit power levels (e.g., 0.1 to2 Watts).

A network controller 130 may couple to a set of eNBs and may providecoordination and control for these eNBs. Network controller 130 maycommunicate with the eNBs via a backhaul. The eNBs may also communicatewith one another, e.g., directly or indirectly via a wireless orwireline backhaul.

UEs 120 may be dispersed throughout wireless network 100, and each UEmay be stationary or mobile. A UE may also be referred to as an accessterminal, a terminal, a mobile station, a subscriber unit, a station,etc. A UE may be a cellular phone, a personal digital assistant (PDA), awireless modem, a wireless communication device, a handheld device, alaptop computer, a cordless phone, a wireless local loop (WLL) station,a tablet, a smart phone, a netbook, a smartbook, etc.

FIG. 2 shows a block diagram of a design of base station/eNB 110 and UE120, which may be one of the base stations/eNBs and one of the UEs inFIG. 1. Base station 110 may be equipped with T antennas 234 a through234 t, and UE 120 may be equipped with R antennas 252 a through 252 r,where in general T≥1 and R≥1.

At base station 110, a transmit processor 220 may receive data from adata source 212 for one or more UEs, select one or more modulation andcoding schemes (MCS) for each UE based on CQIs received from the UE,process (e.g., encode and modulate) the data for each UE based on theMCS(s) selected for the UE, and provide data symbols for all UEs.Transmit processor 220 may also process system information and controlinformation (e.g., CQI requests, grants, upper layer signaling, etc.)and provide overhead symbols and control symbols. Processor 220 may alsogenerate reference symbols for reference signals (e.g., the CRS) andsynchronization signals (e.g., the PSS and SSS). A transmit (TX)multiple-input multiple-output (MIMO) processor 230 may perform spatialprocessing (e.g., precoding) on the data symbols using the PMI(Precoding Matrix Indicator) feedback from the UE, the control symbols,the overhead symbols, and/or the reference symbols, if applicable, andmay provide T output symbol streams to T modulators (MODs) 232 a through232 t. Each modulator 232 may process a respective output symbol stream(e.g., for OFDM, etc.) to obtain an output sample stream. Each modulator232 may further process (e.g., convert to analog, amplify, filter, andupconvert) the output sample stream to obtain a downlink signal. Tdownlink signals from modulators 232 a through 232 t may be transmittedvia T antennas 234 a through 234 t, respectively.

At UE 120, antennas 252 a through 252 r may receive the downlink signalsfrom base station 110 and/or other base stations and may providereceived signals to demodulators (DEMODs) 254 a through 254 r,respectively. Each demodulator 254 may condition (e.g., filter, amplify,downconvert, and digitize) its received signal to obtain input samples.Each demodulator 254 may further process the input samples (e.g., forOFDM, etc.) to obtain received symbols. A MIMO detector 256 may obtainreceived symbols from all R demodulators 254 a through 254 r, performMIMO detection on the received symbols if applicable, and providedetected symbols. A receive processor 258 may process (e.g., demodulateand decode) the detected symbols, provide decoded data for UE 120 to adata sink 260, and provide decoded control information and systeminformation to a controller/processor 280. A channel processor maydetermine RSRP, RSSI, RSRQ, CQI, etc.

On the uplink, at UE 120, a transmit processor 264 may receive andprocess data from a data source 262 and control information (e.g., forreports comprising RSRP, RSSI, RSRQ, CQI, etc.) fromcontroller/processor 280. Processor 264 may also generate referencesymbols for one or more reference signals. The symbols from transmitprocessor 264 may be precoded by a TX MIMO processor 266 if applicable,further processed by modulators 254 a through 254 r (e.g., for SC-FDM,OFDM, etc.), and transmitted to base station 110. At base station 110,the uplink signals from UE 120 and other UEs may be received by antennas234, processed by demodulators 232, detected by a MIMO detector 236 ifapplicable, and further processed by a receive processor 238 to obtaindecoded data and control information sent by UE 120. Processor 238 mayprovide the decoded data to a data sink 239 and the decoded controlinformation to controller/processor 240. Base station 110 may includecommunication unit 244 and communicate to network controller 130 viacommunication unit 244. Network controller 130 may include communicationunit 294, controller/processor 290, and memory 292.

Controllers/processors 240 and 280 may direct the operation at basestation 110 and UE 120, respectively. Processor 240 and/or otherprocessors and modules at base station 110, and/or processor 280 and/orother processors and modules at UE 120, may perform or direct processesfor the techniques described herein. Memories 242 and 282 may store dataand program codes for base station 110 and UE 120, respectively. Ascheduler 246 may schedule UEs for data transmission on the downlinkand/or uplink.

As will be described in further detail below, when transmitting data tothe UE 120 the base station 110 may be configured to determining abundling size based at least in part on a data allocation size andprecode data in bundled contiguous resource blocks of the determinedbundling size, wherein resource blocks in each bundle are precoded witha common precoding matrix. That is, reference signals such as UE-RSand/or data in the resource blocks are precoded using the same precoder.The power level used for the UE-RS in each RB of the bundled RBs mayalso be the same.

The UE 120 may be configured to perform complementary processing todecode data transmitted from the base station 110. For example, the UE120 may be configured to determine a bundling size based on a dataallocation size of received data transmitted from a base station inbundles of contiguous resource blocks (RBs), wherein at least onereference signal in resource blocks in each bundle are precoded with acommon precoding matrix, estimate at least one precoded channel based onthe determined bundling size and one or more reference signals (RSs)transmitted from the base station, and decode the received bundles usingthe estimated precoded channel.

FIG. 3 shows an exemplary frame structure 300 for FDD in LTE. Thetransmission timeline for each of the downlink and uplink may bepartitioned into units of radio frames. Each radio frame may have apredetermined duration (e.g., 10 milliseconds (ms)) and may bepartitioned into 10 subframes with indices of 0 through 9. Each subframemay include two slots. Each radio frame may thus include 20 slots withindices of 0 through 19. Each slot may include L symbol periods, e.g.,seven symbol periods for a normal cyclic prefix (as shown in FIG. 2) orsix symbol periods for an extended cyclic prefix. The 2L symbol periodsin each subframe may be assigned indices of 0 through 2L-1.

In LTE, an eNB may transmit a primary synchronization signal (PSS) and asecondary synchronization signal (SSS) on the downlink in the center1.08 MHz of the system bandwidth for each cell supported by the eNB. ThePSS and SSS may be transmitted in symbol periods 6 and 5, respectively,in subframes 0 and 5 of each radio frame with the normal cyclic prefix,as shown in FIG. 3. The PSS and SSS may be used by UEs for cell searchand acquisition. The eNB may transmit a cell-specific reference signal(CRS) across the system bandwidth for each cell supported by the eNB.The CRS may be transmitted in certain symbol periods of each subframeand may be used by the UEs to perform channel estimation, channelquality measurement, and/or other functions. The eNB may also transmit aphysical broadcast channel (PBCH) in symbol periods 0 to 3 in slot 1 ofcertain radio frames. The PBCH may carry some system information. TheeNB may transmit other system information such as system informationblocks (SIBs) on a physical downlink shared channel (PDSCH) in certainsubframes. The eNB may transmit control information/data on a physicaldownlink control channel (PDCCH) in the first B symbol periods of asubframe, where B may be configurable for each subframe. The eNB maytransmit traffic data and/or other data on the PDSCH in the remainingsymbol periods of each subframe.

In certain systems, higher dimension 3D MIMO (as well as “lowerdimension” 2D MIMO) systems have been discussed to enhance the peak datarate. As an example, in a 2D antenna array system with 64 antennas, itis possible to deploy a grid of 8×8 antennas on a 2D plane, as shown inFIG. 4. In this case, horizontal beamforming as well as verticalbeamforming may used to exploit beamforming/SDMA gain both in azimuthand elevation. 8 antennas at the eNB, deployed in azimuth dimensiononly, allows SDMA or SU-MIMO in horizontal direction. Further inclusionof antennas in elevation, however, allows beamforming also in thevertical plane (e.g. to support different floors in a high risebuilding.

Example Periodic and Aperiodic Channel State Information Reporting

Certain aspects of the present disclosure provide mechanisms for usingdifferent parameters for configuring periodic and aperiodic channelstate information (CSI) reporting. This may allow for reductions infeedback overhead for some CSI reporting.

In some MIMO systems, including 3D MIMO systems, the number of antennaports that may be considered may be constrained by the form factor of abase station. In developing MIMO systems, developing antenna arrays thatcan perform three dimensional beamforming, which may allow forbeamforming horizontally and vertically (i.e., in the azimuth andelevation dimensions), may be desirable. For example, a 2D antenna arraywith a larger number of antenna ports can be designed to allow formulti-user (MU) 3D MIMO.

FIG. 5 illustrates an example 2D antenna port structure that may be usedfor 3D MIMO beamforming. The 2D antenna port structure can have eightantenna ports horizontally and four antenna ports vertically, resultingin a total of 32 antenna ports. Each antenna port may be formed from atwo-element vertical sub-array. Such a sub-array may provide fordirectional antenna gain in the elevation domain.

FIGS. 6A and 6B illustrates examples of steering antenna ports tosupport elevation beamforming and 3D MIMO. In one example, asillustrated in FIG. 6A, the antenna ports may be steered to the samedirection in elevation. This steering may allow for flexible elevationbeamforming and 3D MIMO operations. In another example, as illustratedin FIG. 6B, the elevation domain antennas may be steered to differentdirections. Steering the elevation domain antennas to differentdirections may allow for the formation of multiple elevation domainsubsectors, which may be adapted depending on the location of users andthe traffic load.

In large scale antenna systems, the number of channel responses that aterminal may need to estimate may be proportional to the number oftransmission antenna ports. For implicit CSI feedback using a channelquality indicator (CQI), precoding matrix indicator (PMI), or rankindicator (RI), the complexity involved in determining a CSI report mayincrease exponentially with the number of antenna ports.

The uplink resources needed for CSI feedback may also scale with thenumber of antenna ports. However, because there may be a limitation onthe total number of multiplexing bits (e.g., for periodic CSI feedbackusing PUCCH), CSI may be dropped if the amount of data exceeds the totalnumber of multiplexing bits. For example, for PUCCH format 3, 22 bitsmay be used to multiplex HARQ-ACKs, scheduling request (SR) bits, andthe channel state information, and if the total number of bits forHARQ-ACKs, SR, and CSI exceeds 22 bits, some information may be dropped.

However, in 3D-MIMO, not all antenna ports may be visible to a UE, andnot all antenna ports may be received by a UE with the same signalstrength depending on, for example, how the elevation antenna ports aremapped. However, reporting CSI for weak antenna ports may bemeaningless. Further, for UEs that are close in proximity to a basestation, beamforming with less antenna ports may achieve a sufficientbeamforming gain and capacity. These UEs may not need provide CSIfeedback for beamforming from all transmission antenna ports.

Thus, for 3D-MIMO and a large number of antenna ports, it may not benecessary for a 3D-MIMO capable UE to report CSI for all antenna ports.For example, it may be desirable for UE to not transmit periodic CSIfeedback for all antenna ports using PUCCH.

Antenna selection may be applied for CSI measurement and feedback, forexample, when a large antenna array is used at a base station for 3DMIMO transmissions.

In some cases, a BS may configure the CSI reporting mode to reduce CSIfeedback overhead. For example, if a precoding matrix indicator (PMI) orrank indicator (RI) report (pmi-RI-Report) parameter is not configuredfor CSI reporting, a UE may report only transmission diversity channelquality indicator (TX Div CQI), assuming no precoding, and the UE neednot report PMI and RI. Alternatively, the UE may feedback precoded CQIand antenna port number dependent PMI and/or RI to the eNB. Thesereporting modes imply that transmission beamforming may not be based onfeedback from a UE if a pmi-RI-Report parameter is not configured forCSI reporting. Alternatively, a fewer number of antenna ports may beconfigured for CSI feedback, which may restrict the use of a largenumber of antenna ports for 3D MIMO.

To allow for the use of a large number of antenna ports in 3D MIMO,periodic and aperiodic CSI reporting may be configured with differentCSI parameters. Periodic CSI reporting may provide limited CSI for largenumbers of antenna ports for 3D MIMO operations, with some performancedegradation, and aperiodic CSI reporting using PUSCH may provide fullCSI for all configured antenna ports. Configuring periodic and aperiodicCSI reporting with different parameters may, for example, reduceperiodic CSI feedback overhead on PUCCH. In some embodiments, thepmi-RI-Report parameter for 3D MIMO-capable UEs may be configuredseparately for periodic and aperiodic CSI reporting. For example, thepmi-RI-Report parameter may be configured for aperiodic CSI reportingusing PUSCH but not configured for periodic CSI reporting using PUCCH.In some embodiments, 3D MIMO-capable UEs may be configured for periodicand aperiodic CSI reporting with different numbers of CSI-RS antennaports. For example, the antenna ports configured for periodic CSIreporting may be a subset of the antenna ports configured for aperiodicCSI reporting. The subset may be configured by an eNB (e.g., usinghigher layer signaling) or autonomously by a UE.

Periodic CQI reporting using PUSCH may be configured with the same CSIreporting parameters used for configuring aperiodic CSI reporting.

FIG. 7 illustrates an example of separately configuring a pmi-RI-Reportparameter for periodic and aperiodic CSI reporting. As illustrated, a UEcan be configured with two pmi-RI-Report parameters. An exampleconfiguration may entail reporting CQI alone in periodic CSI reportingand reporting CQI, PMI, and RI in aperiodic reporting. Such aconfiguration may reduce the periodic CSI payload and may cause periodicCSI reporting to not be dependent on the number of CSI-RS antenna ports.

Configuring periodic and aperiodic CSI reporting with differentparameters may, in some embodiments, entail the use of different antennaselections for periodic and aperiodic CSI reporting. Antenna selectionfor CSI feedback may be performed by either a base station or a UE.Regardless of whether antenna selection is performed by a base stationor UE, the UE may be configured with a full set of CSI-RS antenna portswhich may be used for aperiodic CSI reporting.

In an aspect, antenna selection performed by a base station may entailthe following procedure. A BS may transmit a CSI measurementconfiguration message to a UE. The CSI measurement message may comprisea time-frequency resource configuration and a total number of CSI-RSantenna ports that may be used for aperiodic CSI measurement andreporting. Based on uplink received sounding reference signals (SRS),the BS can determine a subset of antenna ports for periodic CSI feedbackfor the UE. The UE may receive a message from the BS indicating thesubset of antenna ports; for example, the message may comprise a bitmapof the antenna ports, where a value of “1” represents an antenna portthat may be used for periodic CSI feedback and a value of “0” representsan antenna port that may not be used for periodic CSI feedback. The UEmay perform channel measurement for the subset of antenna ports. Channelmeasurement may be performed by assuming that downlink data may betransmitted only from the subset of antenna ports, and CSI reporting maybe generated by treating transmissions from antennas outside the subset(i.e., ports not used for periodic CSI feedback) as interference to thesubset of antenna ports. Based on uplink measurements, the BS may updatethe subset of antenna ports selected for periodic CSI feedback.

FIG. 8 illustrates an example of antenna selection performed by a BS forconfiguring and performing periodic and aperiodic CSI reporting by a UE.A first UE may be configured to use a first set of antenna ports (e.g.,antenna ports 0-15), and a second UE may be configured to use a secondset of antenna ports (e.g., antenna ports 16-31) for periodic CSIreporting. The first and second UEs may feedback periodic CSI for theselected antenna ports. Based on CSI feedback and antenna portselection, the BS may perform user pairing and scheduling and mayconstruct beamforming vectors for the base stations. For example, the BSmay construct a beamforming vector for a first UE according to theequation:W ₁=[W ₁₁0]^(T)and a beamforming vector for a second UE according to the equation:W ₂=[0 W ₂₂]^(T),where W₁₁ and W₂₂ comprise precoding vectors determined from theperiodic CSI reporting from the first and second UEs, respectively. Forexample, for a set of 32 antenna ports, W₁₁ and W₂₂ may each comprise16×1 precoding vectors.

In an aspect, a UE may perform antenna selection for periodic andaperiodic CSI reporting. The UE receives a CSI measurement configurationfrom the BS, which may indicate the total number of CSI-RS antenna portsand a time-frequency resource configuration. The UE may measure receivedsignal power for each antenna port and select a set of antenna portsbased on these measurements. For example, for periodic CSI reporting,the UE may select a set of antenna ports with relatively strong receivedpower measurements. The UE may report the measured CSI for the selectedantenna ports and may also report to the BS an indication of the antennaport selection (e.g., a bitmap such as that described above).

FIG. 9 illustrates an example of antenna selection performed by a UE forconfiguring and performing periodic and aperiodic CSI reporting. Anumber of UEs may be configured with a total number of antenna ports.For example, as illustrated, a BS may have a total of 32 antenna portsand may use, for example, multiple elevation subvectors. A first UE maydetermine that a first set of ports have a strongest received signalpower (e.g., ports 0-7) and select the first set of ports for periodicCSI reporting. A second UE may determine that a second set of ports havea strongest received signal power (e.g., ports 16-19 and 28-31) andselect the second set of ports for periodic CSI reporting. The UEs maymeasure and report CSI, including CQI, PMI, and/or RI, of the selectedantenna ports to the BS, as well as an indication of the antenna portselection.

Using a bitmap pattern for feedback of antenna port selection may becostly, as a bitmap pattern may entail the use of a large number of bits(for example, one bit for each antenna). It may be desirable to reducethe amount of resources used to signal antenna selection.

Further, codebooks associated with PMI may be determined byarraystructure. For example, discrete Fourier transform (DFT) based codebookmay be used for closely spaced uniform linear array (ULA) structures.

If arbitrary antenna selection can be used for periodic CSI reporting,multiple codebook sets may need to be defined for different arraystructures. Defining multiple codebook sets may increase complexity. Tominimize increases in complexity, it may be desirable to restrictantenna port selection for periodic CSI reporting.

FIG. 10 illustrates an example of antenna selection options that may beused in a 16-port 2D uniform planar array (UPA). The antenna selectionoptions may result in the 16-port UPA falling back to an 8-port ULAwhere a common codebook structure for the selected eight ports can bedefined. Additionally, the UE need only provide feedback of the periodicCSI port selection, which may comprise a smaller number of bits than abitmap representing the antenna selection. For example, for a 16-portantenna array and four port selection options, feedback of the portselection may comprise two bits rather than 16 bits.

In an embodiment, periodic CSI reporting configuration may entail singlefrequency network (SFN)-like precoding. SFN-like precoding may entaildividing the configured CSI-RS antenna ports into a number of antennagroups. For example, the antenna ports may be divided into groups bycolumn or row of a 2D UPA. Each of the antenna groups may employ thesame precoding, and a UE may select an antenna group by assuming thateach antenna group uses the same precoding and selecting an antennagroup based on the precoding. For example, UE k may report a commonprecoding matrix W_(k) for the B antenna groups. The columns of W_(k)may comprise the L_(k) right singular vectors corresponding to the L_(k)largest singular values of the composite channel:Σ_(b=1) ^(B) H _(k) ^((b)),where H_(k) ^((b)) represents the channel matrix of the b^(th) antennagroup.

FIG. 11 illustrates an example of SFN-like precoding for periodic CSIreporting. Two groups of antenna ports are shown; however, it may berecognized that a BS can configure any number of groups of antennaports. When the BS receives feedback precoding matrices W_(k), the BSmay construct a transmission precoding vector W^((K)) for the antennaports. For example, the transmission precoding vector for the two groupexample illustrated may be represented according to the equation:

$W^{(k)} = {\begin{bmatrix}W_{k} & 0 \\0 & W_{k}\end{bmatrix}.}$The received signal may be represented according to the equation:y _(k)=(Σ_(b=1) ^(B) H _(k) ^((b)) W _(k))x _(k) +n _(k).

FIG. 12 illustrates an example structure for configuring CSI-RSresources. In this example, up to 8 non-zero power (NZP) CSI-RS antennaports can be configured for each CSI-RS resource. The resourceConfigelement may define the resource elements in the frequency domain thatcan be used for CSI-RS transmission, and the subframeConfig element maydefine the subframes in the time domain that can be used for CSI-RS. TheantennaPortsCount element can be extended to support larger CSI-RSantenna port configurations. Extending the antennaPortsCount element mayentail a change to the resourceConfig element, which may be restrictedto a maximum of 8 port mappings to resource elements.

A composite CSI-RS resource with a large number of CSI-RS antenna portscan be constructed from multiple CSI-RS resources having a smallernumber of CSI-RS antenna ports. FIG. 13 illustrates an example structurefor configuring a larger number of CSI-RS ports. Multiple NZP-CSI-RSresources may be aggregated into an NZP-CSI-RS configuration. Forexample, as illustrated, to support a 16 CSI-RS port configuration, twoNZP-CSI-RS resources (e.g., two sets of antennaPortsCount,resourceConfig, and subframeConfig elements) may be aggregated. FIG. 14illustrates another example structure for configuring a larger number ofCSI-RS ports. Multiple NZP-CSI-RS resources may be included in a CSIprocess.

Increasing the size of the NZP-CSI-RS configuration may cause PDSCH ratematching by legacy UEs (i.e., non-3D-MIMO-capable UEs) to not beperformed correctly. Data puncturing may not provide good downlinkperformance if the reference signal overhead per resource block islarge. In some embodiments, providing for successful PDCCH rate matchingby legacy UEs may entail spreading CSI-RS ports to multiple subframes.Spreading CSI-RS ports across subframes may allow for the maintenance ofa small reference signal overhead for each resource block. For example,spreading CSI-RS ports to multiple subframes may be performed such thatthe overhead is less than or equal to 8 resource elements, per resourceblock, per subframe to provide for an acceptable performance impact onlegacy UEs. In some embodiments, configuration of a zero power (ZP)CSI-RS resource can include the resources reserved for NZP-CSI-RS portsnot configured for use by legacy UEs.

FIG. 15 illustrates an example NZP-CSI-RS configuration for a 16 portNZP-CSI-RS configuration. In subframe n, legacy and 3D MIMO UEs canshare the same NZP-CSI-RS resources. In subframe n+1, the NZP-CSI-RSresources for 3D MIMO UEs can overlap with the ZP-CSI-RS resources forlegacy UEs. Thus, legacy UEs can correctly perform PDSCH rate matchingaround the 16 port configuration for 3D MIMO UEs.

FIG. 16 illustrates example operations 1600 that may be performed by abase station (e.g., an eNodeB) to provide for reductions in overhead forCSI reporting, in accordance with aspects of the present disclosure.Operations 1600 may begin at 1602, where a BS configures a UE that iscapable of 3D MIMO with different parameters for periodic and aperiodicCSI reporting, wherein the different parameters indicate at least one ofwhat resources to measure or what information to report. At 1604, the BSreceives periodic and aperiodic CSI reporting from the UE according tothe configuration.

FIG. 17 illustrates example operations 1700 that may be performed by auser equipment to provide for reductions in overhead for CSI reporting,in accordance with aspects of the present disclosure. Operations 1700may begin at 1702, where a UE receives a configuration, from a BS, ofdifferent parameters for periodic and aperiodic CSI reporting, whereinthe different parameters indicate at least one of what resources tomeasure or what information to report. At 1704, the UE measures andreports periodic and aperiodic CSI according to the configuration.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits and algorithm steps described inconnection with the disclosure herein may be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, circuits and steps have beendescribed above generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. Skilled artisans may implement the described functionality invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the present disclosure.

The various illustrative logical blocks, modules, and circuits describedin connection with the disclosure herein may be implemented or performedwith a general-purpose processor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thedisclosure herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and/or write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC. The ASIC may reside in a user terminal. In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal. Generally, where there are operations illustrated inFigures, those operations may have corresponding counterpartmeans-plus-function components with similar numbering.

In one or more exemplary designs, the functions described may beimplemented in hardware, software, firmware or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples and designs described herein, but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method of wireless communications by a basestation (BS), comprising: determining, for different UEs, differentsubsets of antenna ports based on received uplink reference signals;configuring a user equipment (UE) that is capable of MIMO with firstparameters for periodic channel state information (CSI) reporting orsecond parameters for aperiodic CSI reporting, wherein: the first andsecond parameters indicate at least one of what antenna ports to use inmeasuring CSI or what types of CSI to report, the types of CSI to reportcomprise one or more of a channel quality indicator (CQI), precodingmatrix indicator (PMI), or rank indicator (RI), the configurationcomprises an aggregation of information about a plurality of CSIreference signal (CSI-RS) antenna ports on which CSI-RSs are transmittedfor use in measuring CSI for a CSI report, wherein the aggregation ofinformation comprises a first set of information defining properties ofCSI-RS transmission on a first CSI-RS antenna port of the plurality ofCSI-RS antenna ports used for periodic CSI reporting and a second set ofinformation defining properties of CSI-RS transmission on a secondCSI-RS antenna port of the plurality of CSI-RS antenna ports used foraperiodic CSI reporting, the first parameters are different than thesecond parameters, and the configuring comprises configuring the UE tomeasure and report CSI aperiodically for a first set of antenna portsand configuring the UE to measure and report CSI periodically for asecond set of antenna ports; and receiving periodic CSI reporting fromthe UE according to the first parameters or aperiodic CSI reporting fromthe UE according to the second parameters.
 2. The method of claim 1,wherein the UE is capable of 3D MIMO with more than 8 antenna ports. 3.The method of claim 1, wherein the configuring comprises: configuringthe UE to report a first set of information for periodic CSI reporting;and configuring the UE to report a second set of information foraperiodic CSI reporting.
 4. The method of claim 3, wherein: the secondset of information, but not the first set of information, comprisesprecoding matrix indicator (PMI) and rank indication (RI) information.5. The method of claim 4, wherein: the second set of informationcomprises PMI and RI information when CSI is reported aperiodicallyusing a physical uplink shared channel (PUSCH); and the first set ofinformation does not comprise PMI and RI information when CSI isreported periodically using a physical uplink control channel (PUCCH).6. The method of claim 1, wherein the second set of antenna portscomprises a subset of the first set of antenna ports.
 7. The method ofclaim 1, further comprising signaling an indication of the first andsecond sets of antenna ports.
 8. The method of claim 1, furthercomprising determining, for different UEs, different subsets of antennaports based on an indication of selected antenna ports received from theUEs.
 9. The method of claim 8, wherein the indication is used to selectone of a plurality of pre-defined antenna port selection options. 10.The method of claim 1, wherein the configuring comprises: configuringthe UE with a first configuration for CSI reporting for a first set ofports; and configuring the UE with a second configuration for CSIreporting for a second set of ports; wherein a sum of a number of thefirst set of ports and a number of the second set of ports is less thana total number of ports used for CSI reporting.
 11. An apparatus forwireless communications by a base station (BS), comprising: a processorconfigured to: determine, for different UEs, different subsets ofantenna ports based on received uplink reference signals, and generatefirst parameters for periodic channel state information (CSI) reportingor second parameters for aperiodic CSI reporting, wherein: the first andsecond parameters indicate at least one of what antenna ports to use inmeasuring CSI or what types of CSI to report, the types of CSI to reportcomprise one or more of a channel quality indicator (CQI), precodingmatrix indicator (PMI), or rank indicator (RI), the configurationcomprises an aggregation of information about a plurality of CSIreference signal (CSI-RS) antenna ports on which CSI-RSs are transmittedfor use in measuring CSI for a CSI report, wherein the aggregation ofinformation comprises a first set of information defining properties ofCSI-RS transmission on a first CSI-RS antenna port of the plurality ofCSI-RS antenna ports used for periodic CSI reporting and a second set ofinformation defining properties of CSI-RS transmission on a secondCSI-RS antenna port of the plurality of CSI-RS antenna ports used foraperiodic CSI reporting, the processor is configured to configure the UEto measure and report CSI aperiodically for a first set of antenna portsand configuring the UE to measure and report CSI periodically for asecond set of antenna ports, and the first parameters are different thanthe second parameters; and a receiver configured to receive periodic CSIreporting from the UE according to the first parameters or aperiodic CSIreporting from the UE according to the second parameters.
 12. Theapparatus of claim 11, wherein the UE is capable of 3D MIMO with morethan 8 antenna ports.
 13. The apparatus of claim 11, wherein theconfiguring comprises: configuring the UE with a first configuration forCSI reporting for a first set of ports; and configuring the UE with asecond configuration for CSI reporting for a second set of ports. 14.The apparatus of claim 13, wherein: the second set of information, butnot the first set of information, comprises precoding matrix indicator(PMI) and rank indication (RI) information.
 15. The apparatus of claim14, wherein: the second set of information comprises PMI and RIinformation when CSI is reported aperiodically using a physical uplinkshared channel (PUSCH); and the first set of information does notcomprise PMI and RI information when CSI is reported periodically usinga physical uplink control channel (PUCCH).
 16. The apparatus of claim11, wherein the second set of antenna ports comprises a subset of thefirst set of antenna ports.
 17. The apparatus of claim 11, wherein theprocessor is further configured to signal an indication of the first andsecond sets of antenna ports.
 18. The apparatus of claim 11, wherein theprocessor is further configured to determine, for different UEs,different subsets of antenna ports based on an indication of selectedantenna ports received from the UEs.
 19. The apparatus of claim 18,wherein the indication is used to select one of a plurality ofpre-defined antenna port selection options.
 20. The apparatus of claim11, wherein the configuration comprises: configuring the UE with a firstconfiguration for CSI reporting for a first set of ports; andconfiguring the UE with a second configuration for CSI reporting for asecond set of ports; wherein a sum of a number of the first set of portsand a number of the second set of ports is less than a total number ofports used for CSI reporting.
 21. An apparatus for wirelesscommunications by a base station (BS), comprising: means fordetermining, for different UEs, different subsets of antenna ports basedon received uplink reference signals; means for configuring a userequipment (UE) that is capable of MIMO with first parameters forperiodic channel state information (CSI) reporting or second parametersfor aperiodic CSI reporting, wherein: the first and second parametersindicate at least one of what antenna ports to use in measuring CSI orwhat types of CSI to report, the types of CSI to report comprise one ormore of a channel quality indicator (CQI), precoding matrix indicator(PMI), or rank indicator (RI), the configuration comprises anaggregation of information about a plurality of CSI reference signal(CSI-RS) antenna ports on which CSI-RSs are transmitted for use inmeasuring CSI for a CSI report, wherein the aggregation of informationcomprises a first set of information defining properties of CSI-RStransmission on a first CSI-RS antenna port of the plurality of CSI-RSantenna ports used for periodic CSI reporting and a second set ofinformation defining properties of CSI-RS transmission on a secondCSI-RS antenna port of the plurality of CSI-RS antenna ports used foraperiodic CSI reporting, and the first parameters are different than thesecond parameters, and the configuring comprises configuring the UE tomeasure and report CSI aperiodically for a first set of antenna portsand configuring the UE to measure and report CSI periodically for asecond set of antenna ports; and means for receiving periodic CSIreporting from the UE according to the first parameters or aperiodic CSIreporting from the UE according to the second parameters.
 22. Theapparatus of claim 21, wherein the UE is capable of 3D MIMO with morethan 8 antenna ports.
 23. The apparatus of claim 21, wherein theconfiguring comprises: configuring the UE to report a first set ofinformation for periodic CSI reporting; and configuring the UE to reporta second set of information for aperiodic CSI reporting.
 24. Theapparatus of claim 23, wherein: the second set of information, but notthe first set of information, comprises precoding matrix indicator (PMI)and rank indication (RI) information.
 25. The apparatus of claim 24,wherein: the second set of information comprises PMI and RI informationwhen CSI is reported aperiodically using a physical uplink sharedchannel (PUSCH); and the first set of information does not comprise PMIand RI information when CSI is reported periodically using a physicaluplink control channel (PUCCH).
 26. The apparatus of claim 21, whereinthe second set of antenna ports comprises a subset of the first set ofantenna ports.
 27. The apparatus of claim 21, further comprisingsignaling an indication of the first and second sets of antenna ports.28. The apparatus of claim 21, further comprising determining, fordifferent UEs, different subsets of antenna ports based on an indicationof selected antenna ports received from the UEs.
 29. The apparatus ofclaim 21, wherein the indication is used to select one of a plurality ofpre-defined antenna port selection options.
 30. A non-transitorycomputer-readable medium having instructions stored thereon which, whenexecuted by a processor, performs an operation for wirelesscommunications by a base station, the operations comprising:determining, for different UEs, different subsets of antenna ports basedon received uplink reference signals; configuring a user equipment (UE)that is capable of MIMO with first parameters for periodic channel stateinformation (CSI) reporting or second parameters for aperiodic CSIreporting, wherein: the first and second parameters indicate at leastone of what antenna ports to use in measuring CSI or what types of CSIto report, the types of CSI to report comprise one or more of a channelquality indicator (CQI), precoding matrix indicator (PMI), or rankindicator (RI), the configuration comprises an aggregation ofinformation about a plurality of CSI reference signal (CSI-RS) antennaports on which CSI-RSs are transmitted for use in measuring CSI for aCSI report, wherein the aggregation of information comprises a first setof information defining properties of CSI-RS transmission on a firstCSI-RS antenna port of the plurality of CSI-RS antenna ports used forperiodic CSI reporting and a second set of information definingproperties of CSI-RS transmission on a second CSI-RS antenna port of theplurality of CSI-RS antenna ports used for aperiodic CSI reporting, andthe first parameters are different than the second parameters, and theconfiguring comprises configuring the UE to measure and report CSIaperiodically for a first set of antenna ports and configuring the UE tomeasure and report CSI periodically for a second set of antenna ports;and receiving periodic CSI reporting from the UE according to the firstparameters or aperiodic CSI reporting from the UE according to thesecond parameters.